The present invention relates to passenger comfort or climate control systems employed in the passenger compartment of automotive vehicles. Heretofore, where it has been desired to provide a way or means of controlling the discharge air temperature of an automotive heating and air conditioning system, temperature control has been accomplished by varying the opening of the water valve and controlling flow through the heater core or cabin heat exchanger to provide the desired temperature. It has been proposed in co-pending application Ser. No. 248,864 filed Sep. 23, 1988 entitled "Regulating Heater Discharge Air Temperature for Front and Rear Passengers in a Vehicle", commonly owned with the present application to provide automatic control of the heater core water valve by sensing the temperature of the discharge air over the cabin heat exchanger and the position of the valve and comparing with respect to a reference temperature level for generating a control signal to modulate the water valve. This latter type of system functions to regulate the heat exchanger discharge air temperature about a relatively selected level.
However, problems have been experienced in employing the aforesaid automatic discharge air temperature regulation system when the water pump is driven at engine idle speed, because at idle speed there is insufficient flow of heated liquid through the heat exchanger to enable the heat exchanger to provide adequate heat to the discharge air stream as may be needed for the desired regulation. Furthermore, if adequate flow of a liquid is provided at engine idle speed, then at higher engine RPM's encountered at road speeds the water valve is operated in the nearly closed position by the system in order to maintain proper regulation; consequently, it has been necessary for the sensitivity of the system, or resolution of the movement of the water valve to be prohibitively high in order to prevent over controlling or "hunting".
Accordingly, it has been desired to provide a way or means of compensating for variations in speed of an engine driven water pump for controlling liquid flow to a heat exchanger employed in a vehicle passenger compartment.